The present invention relates to a feed-screw unit wherein a reciprocating shaft is reciprocated in accordance with rotation of a rotor, and a method of manufacturing the feed-screw unit. The present invention also relates to an electric motor using the feed-screw unit.
There is known a feed-screw unit wherein a reciprocating shaft is linearly reciprocated by using rotation of a rotor which receives torque. With the feed-screw unit which is used in electric motors as disclosed, e.g. in JP-U 4-108367, the rotor including a female screw is rotatably supported inside a stator coil arranged in a motor housing, whereas the reciprocating shaft including a male screw engaged with the female screw is arranged inside the rotor to axially extend along the rotor.
When the stator coil is controlled by a drive-pulse control signal provided, the rotor is rotated in one direction to move forward an end of the reciprocating shaft for positioning, or in another direction to move forward the end of the reciprocating shaft for positioning. When the rotor is made of a non-metallic material such as a plastic resin, and the reciprocating shaft is made of a metallic material such as steel, positional maintaining of the reciprocating shaft with high accuracy needs to appropriately preserve the mechanical strength of the female screw of the rotor, and to appropriately determine the accuracy of engagement of the female screw of the rotor with the male screw of the reciprocating shaft.
Due to the fact that the female screw of the rotor made of a plastic resin can contract in accordance with the characteristic thereof during cooling after forming, the inner diameter of the female screw should be determined with an appropriate clearance considering the dimension and the contraction coefficient of the male screw which forms a design basis.
Moreover, with manufacturing of the rotor, the reciprocating shaft is replaced with an insert shaft with a male screw in view of the mass-productivity, resulting in necessary consideration of wear of the insert shaft.
As described above, the dimension of the female screw of the rotor is determined considering dispersion of the contraction coefficient of the rotor, wear of the insert shaft, etc., enlarging an engagement clearance between the female screw of the rotor and the male screw of the reciprocating shaft, causing difficult maintaining of the high engagement accuracy. This results in impossible achievement of smooth reciprocation and feed position of the reciprocating shaft with high accuracy.
It is, therefore, an object of the present invention to provide a feed-screw unit wherein a reciprocating shaft is reciprocated in accordance with rotation of a rotor, which enables maintaining of engagement of the reciprocating shaft with the rotor with high accuracy. Another object of the present invention is to provide an electric motor using the feed-screw unit.